Fertilizer reaction is a test for typhoid bacteria. After the body suffers from typhoid fever, antibodies can appear in the body in one to two weeks’ time, and with the prolongation of time, the titer of antibodies will gradually increase, and at this time, the FAT test will be positive, and the titer of antibodies will reach the peak in the fourth week, and then gradually decline thereafter. The mechanism of Fertilizer test is to apply the known typhoid antigen to determine whether there is the corresponding typhoid antibody in the specimen. Although the Fertilizer test is strong evidence of typhoid infection, typhoid cannot be ruled out in patients with a negative Fertilizer test. There are some patients with typhoid fever who have always had a negative Fertilizer test. In view of the possibility of false-negative FAT test, the diagnosis of typhoid fever should be combined with clinical tests and clinical symptoms to make a comprehensive judgment.